Whereas ornithischian dinosaurs are well known from Jurassic and Cretaceous deposits, deciphering the origin and early evolution of the group remains one of the hardest challenges for palaeontologists. So far, there are no unequivocal records of ornithischians from Triassic beds. Here, we present an alternative evolutionary hypothesis that suggests consideration of traditional ‘silesaurids' as a group of low-diversity clades representing a stem group leading to core ornithischians (i.e. unambiguous ornithischians, such as Heterodontosaurus tucki). This is particularly interesting because it fills most of the ghost lineages that emerge from the Triassic. Following the present hypothesis, the lineage that encompasses the Jurassic ornithischians evolved from ‘silesaurids' during the Middle to early Late Triassic, while typical ‘silesaurids' shared the land ecosystems with their relatives until the Late Triassic, when the group completely vanished. Therefore, Ornithischia changes from an obscure to a well-documented clade in the Triassic and is represented by records from Gondwana and Laurasia. Furthermore, according to the present hypothesis, Ornithischia was the first group of dinosaurs to adopt an omnivorous/herbivorous diet. However, this behaviour was achieved as a secondary step instead of an ancestral condition for ornithischians, as the earliest member of the clade is a faunivorous taxon. This pattern was subsequently followed by sauropodomorph dinosaurs. Indeed, the present scenario favours the independent acquisition of an herbivorous diet for ornithischians and sauropodomorphs during the Triassic, whereas the previous hypotheses suggested the independent acquisition for sauropodomorphs, ornithischians, and ‘silesaurids'.

虽然鸟臀目恐龙在侏罗纪和白垩纪的沉积物中广为人知，但破译该类群的起源和早期进化仍然是古生物学家面临的最艰巨的挑战之一。到目前为止，还没有关于三叠纪床的鸟臀目动物的明确记录。在这里，我们提出了另一种进化假说，该假说建议将传统的“西里龙科”视为一组低多样性进化枝，这些进化枝代表导致核心鸟臀目（即明确的鸟臀目，如塔基异齿龙））。这特别有趣，因为它填补了三叠纪出现的大部分幽灵血统。根据目前的假设，侏罗纪鸟臀目的谱系从中到晚三叠世早期的“西里龙科”演化而来，而典型的“西里龙科”则与它们的亲属共享陆地生态系统，直到三叠纪晚期，该群体完全消失。因此，鸟臀目在三叠纪从一个晦涩的进化枝转变为一个有据可查的进化枝，并以冈瓦纳纪和劳亚纪的记录为代表。此外，根据目前的假设，鸟臀目是第一个采用杂食性/草食性饮食的恐龙。然而，这种行为是作为次要步骤实现的，而不是鸟臀目动物的祖先条件，因为该进化枝的最早成员是食性分类群。这种模式随后被蜥脚类恐龙效仿。事实上，目前的情况有利于在三叠纪期间独立获取鸟臀目和蜥脚类动物的食草食物，而之前的假设表明蜥脚类动物、鸟臀类动物和“西里龙科”独立获取食草食物。